Epoxy coating materials are well known and have gained commercial acceptance as protective and decorative coatings for steel, aluminum, galvanized steel and concrete in maintenance, marine, construction, architectural, aircraft and product finishing markets. Epoxy-based coatings possess many properties which make them desirable as coating materials. They are readily available and are easily applied by a variety of methods including spraying, rolling and brushing. They adhere well to steel, concrete and other substrates, have low moisture vapor transmission rates and act as barriers to water, chloride and sulfate ion ingress, provide excellent corrosion protection under a variety of atmospheric exposure conditions and have good resistance to many chemicals and solvents. The basic raw materials used to prepare these coatings generally comprise as essential components (a) an epoxy resin, (b) a hardener and (c) a pigment or filler component. Epoxy-based coatings generally show excellent protective properties but a considerable drawback is the limited gloss and color retention when atmospherically exposed.
Increasing awareness about environment and health and safety for human beings calls for extra attention in the preparation of safe paints. Increasing strictness of environmental rules demand a decrease of the emission of harmful solvents. Paint systems with low VOC contents are required. A currently world-wide wanted VOC level is VOC<250 g/L with respect to application viscosity.
Several paint manufacturers recently developed high solids (HS) paint systems with lower VOC contents to cope with environmental rules and regulations. Examples of these coatings in this respect are the commercially widely accepted protective and decorative isocyanate-cured polyurethane coatings. The 2-pack isocyanate-cured polyurethane coatings combine a high potential in gloss and color retention, good chemical resistance and good mechanical properties. Nowadays in some countries, the use of isocyanates is not allowed anymore because of safety and health regulations. There is an urge to replace these isocyanate cured polyurethane coatings by non-isocyanate coatings (NISO). However, from high to medium VOC content NISO coatings commercially available at present perform considerably less compared to the polyurethane finishes.
Using the common binder technologies to reformulate the existing NISO coatings into low VOC versions has been shown to be very difficult, as it requires the development of new polymers and curing processes to accomplish this task.
There is an urgent need for NISO coatings having low VOC content, good flexibility and the mechanical and chemical resistance properties of epoxy or polyurethane based paintings.
Epoxy-polysiloxane based compounds are known from U.S. Pat. Nos. 5,618,860 and 5,275,645, which describe 2-pack glossy epoxy-polysiloxane compositions. Although having good gloss and color retention properties and having very good chemical resistance they tend to be rather hard and brittle. As finish coating on complex structural steel where a higher flexibility is required they are not suitable. Besides, the shrink stresses during curing are generally too high for application on weaker substrates like concrete and low cohesive coating.
Thus, while epoxy-polysiloxane based coating materials have gained commercial acceptance, the need nevertheless remains for epoxy-polysiloxane based materials with improved mechanical and chemical resistance, and more in particular improved resistance to mechanical abuse. Coating materials with improved chemical, corrosion, impact and abrasion resistance are needed for both primary and secondary chemical containment structures, for protecting steel and concrete in chemical, power generation, rail car, sewage and waste water treatment, and paper and pulp processing industries.
In particular known epoxy-polysiloxane coatings show a tendency to crack on complex steel structures. Cracking tendency occurs frequently at local areas, such as corner joints where higher film thicknesses are mainly present.
A main object of the present invention is therefore to provide an epoxy-modified polysiloxane coating composition, which is high solid, has low VOC content and is NISO. It is another object to provide an epoxy-modified polysiloxane composition having improved flexibility and which does not show this cracking tendency, without compromising other properties like chemical or mechanical resistance and hardness development.